A Class-D power amplifier employs two active switching devices as a single-pole double-throw switch for generating a square wave signal. An output load network includes a resonant circuit which is tuned to the switching frequency of the devices, thus removing the harmonics of the square wave signal and resulting in a sinusoidal output signal. There are two types of Class-D power amplifiers, a voltage-switching type and a current-switching type. In the voltage-switching type, the switching devices are connected in series with a power supply, and the supply voltage is switched alternately between the respective inputs thereof. The current-switching type, which is the dual of the voltage-switching type, employs the devices in a parallel or push-pull configuration, and the supply current is alternately switched therebetween. For either type, since the switching devices of Class-D power amplifiers are driven alternately between cutoff and saturation such that one is conducting while the other one is turned off and vice versa, Class-D power amplifiers are conveniently driven by square wave signals. An input isolation transformer is conventionally used to provide the two out-of-phase driving signals.
Switching power losses at low frequencies are generally negligible for Class-D power amplifiers. However, at higher operating frequencies, switching losses are significant, thus decreasing efficiency. In particular, for a voltage-switching amplifier, the power dissipated by each switching device in discharging the output capacitance of the other switching device increases with increasing operating frequency. Moreover, as frequency increases, isolation transformers which are capable of passing the higher harmonics of the square wave drive signals must be more complex. It is generally accepted that the aforesaid shortcomings impose an upper limit on attainable efficiency for Class-D power amplifiers, e.g. 80% for high frequency, low power applications and less than 70% for high frequency, high power applications.